The present invention relates to optical gap measuring tool calibration. More specifically, the invention relates to a system and method for calibrating a hard disc drive magnetic head flying height tester by optical interference techniques.
FIG. 1 provides an illustration of a typical hard disc drive. In the art of hard disc drives, magnetic read/write heads 102 are commonly integrated in a slider 102 designed to respond to a flow of air moving with the rotating disc 104 over which the slider 102 travels. The head/slider 102 ‘flies’ close to the surface of the disc 104. In manufacturing such heads/sliders 102, it is often necessary to test hydrodynamic characteristics of the heads 102 to verify their performance. It is important that the head 102 not travel too far from or close to the disc 104 surface. Further, it is important to prevent the head 102 from traveling at an improper angle with respect to the disc surface 104. A head 102 traveling too high above the disc surface 104 will result in a lower than desired areal density. A head 102 traveling too low can cause an interface failure between the head 102 and disc 104.
In order to test the flying height of the head, a flying height tester is commonly used. Optical interference techniques are often employed to determine the distance between head and disc. A monochromatic light source is directed at a transparent surrogate disc, such as a glass disc, rotating at speeds similar to that of a magnetic disc, and the head assembly being tested is secured in a holder in its normal flying orientation in relation to the disc. The monochromatic light is directed at the disc at a predetermined angle to the surface thereof. The light is reflected from the surface of the disc closest to the head, as well as from the surface of the flying head itself, and impinges onto a light sensitive sensor.
The interference effects created by the combined reflections from the disc and the slider surface provide the flying height information. A computer receives data from the flying height tester and calculates the perceived flying height and angle of the head. As hard drives become smaller and increase in data storage capacity, the desired head flying height continually reduces. Therefore, the accuracy of a flying height tester, and thus its calibration, are of critical concern.
FIG. 2 illustrates a typical device used to calibrate a flying height tester. A calibration standard, such as is depicted in U.S. Pat. No. 5,552,884, is often utilized. As can be seen in FIG. 2a, the calibration standard includes a mock head 48 in contact with a transparent disc 44 via a load spring 52. The transparent disc 44 has a plurality of grooves 60 formed in a surface facing the mock head 48. A cover case 56 is attached to the glass disc 44 at one end and provides a sealed environment for the interface between the mock head 48 assembly and the transparent disc 44. Several problems exist with the utilization of this device. For example, in establishing H1 204, which is important in evaluating flying height (explained below), the nature of the design causes problems with using optical interference means. Measurement of H1 205 must not be taken too close to a ridge's 64 edge, or else one (or both) of the measurement light beam's return paths 206,208 may travel a portion through air (separated by the walls at 120 and 124). The differences in optical properties between air and the transparent disc (glass, etc.) disrupts the travel path and thus causes inaccurate optical interference measurement results (i.e., the resultant beams 206 and 208 are not at the correct positions and/or the correct distance apart for accurate measurement). Therefore, H1 measurements may only be taken towards the center of the ridges 64 (if at all). This prevents appropriate compensation for surface irregularities 76 in the mock disc 48. Also, a separate device must be used to determine a minimum and maximum light intensity for the flying height tester, a necessary step in calibration, as explained below. This separate device adds cost and complexity to the calibration process.
It is therefore desirable to have a system and method for calibrating flying height testers that avoids the above-mentioned problems, as well as having additional benefits.